1. Field of the Invention
The present invention relates to a device and method for indicating substantially instantaneously the bandwidth of an electrical signal having several frequency components, that is, a "broadband" electrical signal, and particularly to such a device and method for indicating the bandwidth of the output of a medical Doppler device.
2. Prior Art
Medical Doppler devices are used to detect the velocity and volume of blood flowing through a blood vessel. In general, ultrasound waves of a known frequency are beamed into the body by a transmitting transducer held stationary relative to the blood vessel. A receiving transducer stationary relative to the transmitting transducer detects the waves reflected from blood moving through the blood vessel. The difference in frequency of a wave before and after its reflection indicates the velocity of the blood from which the wave was reflected. The medical Doppler device generates a broadband, audio frequency, electrical output signal; and the relative amplitude of each frequency component of such output signal indicates the proportion of blood flowing at the corresponding velocity.
Signal-analyzing devices are known which display the audio frequency output signal as a trace on a cathoderay tube. Such a device can indicate the average blood velocity over time. Comparison of the Doppler output signal before and after a heartbeat as represented by the trace may suggest the condition of the heart or the condition of the blood vessel, or the transducers can be moved incrementally along a blood vessel extending generally parallel to the surface of the body to detect or locate a partial blockage.
More sophisticated signal-analyzing devices known as spectrum analyzers use digital and Fourier transform techniques to detect the individual frequency components of the Doppler output and to calculate the relative amplitudes of such components. Various types of displays can be generated to indicate the distribution of Doppler output frequencies, which corresponds to the distribution of blood velocities. For example, a video display can show a narrow or broad band pattern indicating the range of frequency components detected over time. A desired time in the heart cycle can be selected by a cursor, and a second display resembling a bar graph or histogram can indicate the amplitude of each frequency component at the selected time.
Turbulent flow through the blood vessel can be indicated by a wide range of velocities at a given point in time which, in turn, is indicated by a Doppler output of a wide frequency range. One measure of the frequency range is the "bandwidth" which has been defined as the difference between the upper and lower half-power frequencies. For turbulent flow, however, not only is a broad range of frequencies present in the Doppler output but also the amplitudes of the various frequency components can vary irregularly and may not be symmetrical with respect to any mid or maximum power frequency. Half-power bandwidth may not give a reliable, diagnostically valuable indication of the frequency distribution because Doppler output signals having quite different frequency distributions may, nevertheless, have about the same half-power bandwidth.
Medical Doppler devices and/or signal-analyzing devices described as being for use with medical Doppler devices are described in the following U.S. patents:
McLeod, Jr. U.S. Pat. No. 3,430,625;
Shaw et al. U.S. Pat. No. 3,498,290;
Light U.S. Pat. No. 3,710,792;
Flaherty et al. U.S. Pat. No. 3,732,532;
Sato U.S. Pat. No. 3,896,788;
McCarty et al. U.S. Pat. No. 3,901,077;
Groves et al. U.S. Pat. No. 3,922,911;
Djordjevich U.S. Pat. No. 3,996,925;
Aronson U.S. Pat. No. 4,103,679;
Papadofrangakis U.S. Pat. No. 4,217,909;
Papadofrangakis et al. U.S. Pat. No. 4,257,278; and
Papadofrangakis et al. U.S. Pat. No. 4,265,126.